The quickest way (and really the only way) to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter (5W). When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge very slowly (but iPad indicates not charging). Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Apple recommends that once a month you let the iPad fully discharge & then recharge to 100%.
How to Calibrate Your Mac, iPhone, or iPad Battery
At this link http://www.tomshardware.com/reviews/galaxy-tab-android-tablet,3014-11.html , tests show that the iPad 2 battery (25 watt-hours) will charge to 90% in 3 hours 1 minute. It will charge to 100% in 4 hours 2 minutes. The new iPad has a larger capacity battery (42 watt-hours), so using the 10W charger will obviously take longer. If you are using your iPad while charging, it will take even longer. It's best to turn your new iPad OFF and charge over night. Also look at The iPad's charging challenge explained http://www.macworld.com/article/1150356/ipadcharging.html
Also, if you have a 3rd generation iPad, look at
Apple: iPad Battery Nothing to Get Charged Up About
Apple Explains New iPad's Continued Charging Beyond 100% Battery Level
New iPad Takes Much Longer to Charge Than iPad 2
Apple Batteries - iPad http://www.apple.com/batteries/ipad.html
Extend iPad Battery Life (Look at pjl123 comment)
New iPad Slow to Recharge, Barely Charges During Use
Tips About Charging for New iPad 3
Prolong battery lifespan for iPad / iPad 2 / iPad 3: charging tips
In rare instances when using the Camera Connection Kit, you may notice that iPad does not charge after using the Camera Connection Kit. Disconnecting and reconnecting the iPad from the charger will resolve this issue.
below is the link to asus ai charger, it does actually works and the software is free. it cuts the charging time to half in my pc. btw, before the installation of this software my ipad 3 indicates "not charging" but after the installation charging went smooth, same charging time using wall charger.
Lots of users that come here already know the answers many wiseasses here are going to give. Here is an observation; anything that I plugged to the iPad for power usually says "not charging". It is almost like standard protocol for the iPad to say "not charging" if one is not using Apple's very own cable. I know not all cables will say that but most and I mean MOST do. Regardless, any interface seems to charge it. My gray charger does it. It says "not charging" but it increased about 15% charge within an hour. It is slow but it works. So, do not think it will not charge because it says "not charging"; it will charge but in a painfully slow speed. If you use the iPad while charging with these systems, than power consumption will be faster than what it charges. Therefore, if you want to charge a lot faster with no problems, then you are better of buying their 10w charger.
Lots of users that come here already know the answers many wiseasses here are going to give. Here is an observation; anything that I plugged to the iPad for power usually says "not charging".
So to summarise: when choosing a charger, get one that offers sufficient power. As in two amps.